Telecommunication systems that interconnect wire line subscriber terminals are being developed to support broadband data communication. More particularly, recent developments in broadband communication protocols allow broadband data to be overlaid on narrowband voice or integrated service digital network (ISDN) traffic. Specifically, the interconnection of broadband modems located at the subscriber terminal and at an exchange allow current broadband access systems to communicate on spare spectrum (i.e. spare frequency channels) of a twisted pair communication resource; the spare frequency channels being isolated from conventionally encoded voice signals by a suitable filter. In this respect, and depending upon the complexity of the xDSL coding scheme, overlaid broadband systems can support data rates in excess of two Megabits per second (Mbps), although this rate is dependent upon the physical parameters of the connection, e.g. the overall length of the twisted pair and its composition and configuration.
Asymmetric Digital Subscriber Line (ADSL) and High-speed Digital Subscriber Line (HDSL) protocols, for example, can support data rates of 2 Mbps over distances of approximately three kilometers, while more complex schemes (such as VDSL) can support data rates of 8 Mbps and above over distances of, typically, less than two kilometers. Line codes such as discrete multi-tone (DMT), which can be used for Very high-speed Digital Subscriber Line (VDSL), utilise multiple sub-channel carriers, e.g. in a DMT environment, to provide an adaptive system that mitigates the effects of cross-talk by selectively ignoring noise-affected sub-channel carriers or reducing the number of bits supported by the sub-channels. As will be appreciated, DMT provides a comb of frequency carriers that are each separated modulated and then combined to generate a composite signal envelope. As such, information (both control information and traffic) is distributed across a number of different frequency carriers.
Presently, some xDSL systems (and the like) utilise a time division duplex transmission scheme in which a communication resource (such as a dedicated channel within frequency limits) has a time-split use for up-link and down-link transmissions between line termination equipment (LTE) and customer premises equipment (CPE). More specifically, the up-link and down-link may have different traffic capacities, i.e. there is a fixed but potentially asymmetric symbol capacity (or number of time slots) between the up-link and the down-link assigned for the entire duration of a call. For example, in an Internet-type environment, it is usually beneficial to have a higher down-link capacity since information download is the dominant data flow, whereas voice traffic generally requires equal traffic capabilities in both directions.
In frequency division duplex (FDD) systems, spectrum is allocated between the up-link and down-link.
In relation to bundles of wireline communication resources, it is also important to consider the potentially undesirable effects associated with cross-talk interference. Specifically, with bidirectional communication, the relative location of the lines, for example, between twisted copper-pair causes cross-talk interference to be induced into proximately located wireline communication resources (principally by the mechanisms of capacitive and inductive coupling and by radiation arising from the imperfect nature and performance of the cabling). Moreover, where symmetrical and asymmetrical service are simultaneously required on pairs in the same bundle, cross-talk becomes a significant problem, as will readily be appreciated.
It is known in such systems to use an initialisation training sequence to identify connection parameters for use in transmitting and receiving subsequent user data.
Such systems include, for example, those described in ITU-T pre-published Recommendations G.992.1 “Asymmetrical Digital Subscriber Line (ADSL)” and G.992.2 “Splitterless Asymmetric Digital Subscriber Line (ADSL) Transceivers”. ITU-T Recommendation G.994.1 “Handshake Procedures for Digital Subscriber Line (DSL) Transceivers”.
A problem with the negotiation protocols used in such systems is that they can be unreliable owing to, amongst other causes, the presence of increasing levels of loss of data owing to unacceptably high signal attenuation and consequent low Signal-to-Noise Ratio (SNR) particularly on longer connections between headend and subscriber.
A known approach to try to address these problems is to send two simultaneous copies of each symbol, each copy being sent over a separate, pre-defined group of carriers. For an 8 bit/symbol encoding, QPSK modulated, such a group consists of 4 adjacent carriers, each carrying 2 bits of the symbol. The intention is that if one group fails to be received, the symbol may nevertheless be recovered, by the receiver, from the second backup group.
However, it is not uncommon for the backup group also to fail, thereby causing failure of the communication, as a whole, requiring the initialisation sequence to be re-started.
An alternative approach, still retained for the most critical initialisation information, involves using a reduced bit/symbol encoding, for example 1 bit/symbol.
A problem here however is that such encodings, whilst more reliable individually, require longer transmission times per message, thereby unacceptably lengthening initialisation times.
It is also known, from ITU-T Study Group 15 Temporary Document HC-034 to replace the use of a pre-defined pair of groups by a dynamically negotiated group, whereby account can be taken of the frequency distribution of the channel SNR.
This has the added disadvantage however, of extra complexity and of requiring transmission of additional symbols communicating the selection of the specific negotiated channel carriers forming the groups, thereby once again undesirably extending the training time.